An air conditioner is known in the related art, which ham a refrigeration cycle created by using refrigerant piping to connect an outdoor unit that includes a compressor, an outdoor heat exchanger, an outdoor fan and the like, to an indoor unit that includes an indoor heat exchanger and the like, in which cooling, heating and/or the like are provided through the indoor unit. If the compressor used in such an air conditioner sucks a large amount of liquid refrigerant, the compressor may possibly suffer a breakdown. Therefore, to avoid liquid back to the compressor, the opening control of an expansion valve (e.g., an electronic expansion valve) installed in the refrigeration cycle is important.
In techniques known as the opening control of the expansion valve, the discharge temperature of the compressor is controlled to reach a predetermined value, and/or, the degree of refrigerant superheat at the outlet of the heat exchanger which is an evaporator is controlled to reach a predetermined value.
Techniques disclosed in the Japanese Patent Application Laid-Open No. 2004-225924 (Patent Literature 1) include: the control of expansion means such that the degree of superheat at the outlet of the heat exchanger which is an evaporator is set at zero and also the degree of refrigerant dryness becomes one; the control of expansion means such that an evaporation temperature is obtained by conversion from low pressure and the evaporation temperature is caused to approach a target evaporation temperature; the control of expansion means such that the degree of refrigerant superheat at the outlet of the compressor falls within a predetermined range; and/or the like.
Further, a control method is disclosed in the Japanese Patent Application Laid-Open No. 2002-327950 (Patent Literature 2), in which, for control of the opening degree of an indoor expansion valve such that the degree of refrigerant superheat at the evaporator outlet reaches a desired value and for control of the capacity of the compressor such that the evaporation temperature of the refrigerant in the evaporator reaches a target temperature, a dynamic model is used to improve the mutual destabilizing influence of the controls on the evaporation temperatures in both the controls.